1. Field of the Invention
This invention relates to a process for dewaxing of heavy oils using a large pore zeolite which has been modified by coke deposition.
2. Description of Relevant Art
Selective dewaxing of hydrocarbon oils, both for pour point improvement and for preparation of lubricating oil base stocks, is well-known.
Much work has been done on catalytic hydrodewaxing using shape selective zeolites.
The ideal hydrodewaxing catalyst should have several necessary chemical and physical features.
The zeolite should have a pore size which is large enough to admit waxy paraffins and slightly branched paraffins but small enough to exclude, or diffuse only slowly, "non-paraffinic" oil molecules. The relative rates of diffusion of waxy and non-waxy components should be sufficiently different so as to favor selective conversion of the waxy paraffinic molecules.
Some zeolites stand out for hydrodewaxing and many other hydroconversion processes; these are the shape selective zeolites, such as ZSM-5 (U.S. Pat. No. 3,702,886). ZSM-5 has a highly stable framework containing two types of intersecting channels which have ten-membered ring openings. These are therefore intermediate between those of classical shape-selective zeolites with 8 membered rings (zeolite A, erionite) and the larger pore 12 membered ring zeolites (faujasite, X, Y, mordenite and fault free offretite).
ZSM-5 has two sets of channels, in one direction the channels are sinusoidal with near circular openings of about 0.55 nm. The other channels are straight with elliptical cross section (0.52-0.58 nm) Nature 272, 437 (1978). Further the zeolite can be crystallized with very high SiO.sub.2 contents giving it hydrophobic properties. ZSM-5 has a Constraint Index, as hereafter defined, of about 8.
Some work has been done on modifying the diffusivity characteristics of zeolites with a Constraint Index of 1 to 12. U.S. Pat. No. 4,117,026, the entire contents of which are incorporated herein by reference, discloses one such process. The shape selective zeolites are modified by combining therewith a small amount, generally 2 to 30 weight percent, of a difficultly reducible oxide, such as oxides of antimony, boron, etc. Modification of the zeolite by precoking to deposit at least one weight percent of coke is disclosed.
U.S. Pat. No. 4,097,543, the entire contents of which are incorporated herein by reference, discloses controlled precoking of shape selective zeolites, i.e., those having a Constraint Index of 1 to 12. The catalyst is used for selective production of paraxylene by disproportionation of toluene.
Closely related to, and an improvement upon, the teachings of U.S. Pat. No. 4,097,543 and U.S. Pat. No. 4,117,026 is application Ser. No. 524,626, assigned to the same assignee. This application discloses carefully controlled coking of a zeolite with a Constraint Index of 1-12.
Large pore zeolites having a Constraint Index less than 1, are generally regarded as not being shape selective, although both mordenite and offretite are selective enough for certain hydrodewaxing operations. Typical large pore zeolites unsuitable for dewaxing are X and Y, and structurally related zeolites such as ZSM-3 and ZSM-20.
Zeolite Y is one of the largest known 12 ring zeolites and is used in hydrocracking applications where hydrocarbon molecules of various types and shapes are converted. One useful feature of this type of zeolite is that it possesses the structure of the rare mineral faujasite. It has a 3 dimensional network of openings connecting supercages. This open structure is characterized by the zeolite's high capacity for the sorption of hydrocarbons such as cyclohexane, i.e. &gt;10 wt. % at 25.degree. C.
Zeolite X is another large, and well-known faujasite type zeolite. Zeolite X is a known hydrocracking catalyst, typically used in a rare earth exchanged form and contained in amorphous binder, typically with hydrogenation/dehydrogenation components added.
Some work has been done on modifying the characteristics of certain large pore zeolites (mordenite, offretite and zeolite Y) by silylation, as noted in U.S. Pat. No. 4,390,414, the entire contents of which are incorporated herein by reference. This patent includes a discussion of prior art silylation work.
U.S. Pat. No. 4,231,899, the entire contents of which are incorporated herein by reference, teaches stabilizing intermediate pore sized zeolites against steam deactivation by coking the zeolite. It was recognized that the coked zeolite had a reduced sorption capacity, indicating that some carbonaceous residue was within the zeolitic pores. The coking process was useful for adding steam stability to zeolites, such as ZSM-5, ZSM-11, ZSM-12. This reference taught that materials such as erionite, mordenite and faujasite, having a higher constraint index (or larger pore size) than ZSM-5 may also be subjected to coking to impart steam stability. No examples of coking of large pore zeolites were provided. The thrust of the invention was to develop a zeolite which would be steam stable, i.e., one which would withstand the large amounts of steam encountered in converting methanol to hydrocarbon products. Intermediate pore zeolites, such as ZSM-5, are used in such processes rather than large pore zeolites.
There was no indication as to the effect of the coke deposited upon the zeolites' shape selectivity. A loss of sorption capacity due to coking was noted, however the effect of coke on hydrophobicity is not mentioned.
A certain amount of coke deposition gradually occurs in conventional hydrocracking processes. In general, coke deposition in hydrocracking is avoided. Coke deposition covers up active sites upon the catalysts. This phenomenon was also recognized in the previously discussed U.S. Pat. No. 4,231,899, which taught that coke covered sites are not catalytically active until the coke is removed.
Accordingly the art has generally considered coke deposition as something to be avoided, and taught that when it does occur catalytically active sites covered by the coke are rendered inactive.
We discovered that is was possible, by coke deposition in the pores of a large pore zeolite with cyclohexane sorption capacity of &gt;10%, to produce a zeolite which exhibited shape selectivity, i.e., which behaved as if it had a smaller pore size, equivalent to a constraint index greater than one. There was some loss of catalytic activity, but careful control of the large pore zeolite starting material, and coke deposition operating conditions, permitted shape selective catalytic dewaxing with a large pore zeolite.